1. Field of the Invention
The present invention relates to an organic optoelectrical device and a method of manufacture thereof.
2. Related Technology
One class of optoelectrical devices uses organic material for light emission (or detection in the case of photovoltaic cells and the like). The basic structure of these devices comprises a light emissive organic layer, for instance a film of a poly(p-phenylene vinylene) (“PPV”) or polyfluorene, sandwiched between a cathode for injecting negative charge carriers (or electrons) and an anode for injecting positive charge carriers (holes) into the organic layer. The electrons and holes combine in the organic layer generating photons. In WO 90/13148 the organic light emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light emissive material is of the class known as small molecule materials, such as (8-hydroxyquinolene) aluminum (“ALq3”). In a practical device one of the electrodes is transparent, to allow the photons to escape the device.
Typically, the above-described devices comprise: a substrate; a first electrode disposed over the substrate for injecting charge of a first polarity; a second electrode disposed over the first electrode for injecting charge of a second polarity opposite to said first polarity; an organic light emitting layer disposed between the first and the second electrodes; and an encapsulant disposed over the second electrode. In one arrangement, the substrate and the first electrode are transparent to allow light emitted by the organic light emitting layer to pass therethrough. In another arrangement, the second electrode and the encapsulant are transparent so as to allow light emitted from the light emitting layer to pass therethrough.
Variations of the above-described structures are known. The first electrode may be the anode and the second electrode may be the cathode. Alternatively, the first electrode may be the cathode and the second electrode may be the anode. Further layers may be provided between the electrodes and the organic light emitting layer in order to improve charge injection and transport. The organic material in the light emitting layer may comprise a small molecule, a dendrimer or a polymer and may comprise phosphorescent moieties and/or fluorescent moieties. The light emitting layer may comprise a blend of materials including light emitting moieties, electron transport moieties and hole transport moieties. These may be provided in a single molecule or on separate molecules.
By providing an array of devices of the type described above, a display may be formed comprising a plurality of emitting pixels. The pixels may be of the same type to form a monochrome display or they may be different colors to form a multicolor display.
To control the pixels, and so form the image required, either “passive” or “active” matrix driver methods are used.
Active matrix displays incorporate a transistor (TFT) in series with each pixel which provides control over the current and hence the brightness of individual pixels. Lower currents can flow down a control wire since these have only to program the TFT driver, and the wires can be fine as a result. Also, the transistor is able to hold at the current setting, keeping the pixel at the required brightness, until it receives another control signal. DC drive conditions typically are used for an active matrix display.
In passive matrix systems, the matrix is rapidly scanned to enable every pixel to be switched on or off as required. The controlling current has to be present whenever the pixel is required to light up.
In order to form the pixels, a bank layer is deposited over the first electrodes and patterned by, for example, standard photolithographic techniques, in order to define wells in which the organic emitting material can be deposited. The material used for the bank layer is usually an organic material such as a polyimide. Additionally, cathode separators may be formed on the blank structure, e.g. resist, polyimide.
FIG. 1 shows a vertical cross-section through an example of an organic light-emitting diode (OLED) device. The structure of the device is somewhat simplified for the purposes of illustration.
An OLED 100 comprises a substrate 102, typically 0.7 mm to 1.1 mm glass but optionally clear plastic, on which an anode layer 106 has been deposited. The anode layer typically comprises around 150 nm thickness of ITO (indium tin oxide), over which is provided a metal contact layer, typically around 500 nm of aluminum, sometimes referred to as anode metal. Glass substrates coated with ITO and contact metal may be purchased from Corning, USA. The contact metal (and optionally the ITO) is patterned as desired so that it does not obscure the display, by a conventional process of photolithography followed by etching.
A substantially transparent hole transport layer 108a is provided over the anode metal, followed by an electroluminescent layer 108b. Banks 112 may be formed on the substrate, for example from positive or negative photoresistant material, to define wells 114 into which these active organic layers may be selectively deposited, for example by a droplet deposition or inkjet printing techniques. The wells thus define light emitting areas or pixels of the displays.
A cathode layer 110 is then applied by, for example, physical vapor deposition. The cathode layer typically comprises a low work function metal such as calcium or barium covered with a thicker, capping layer of aluminum and optionally including an additional layer immediately adjacent the electroluminescent layer, such as a layer of lithium fluoride, for improved electron energy level matching. Mutual electrical isolation of cathode lines may be achieved through the use of cathode separators (element 302 of FIG. 3b). Typically a number of displays are fabricated on a single substrate and at the end of the fabrication process the substrate is scribed and the displays separated. An encapsulant such as a glass sheet or a metal can is utilized to inhibit oxidation and moisture ingress.
FIG. 2 shows a view from above (that is, not through the substrate) of a portion of a three color active matrix pixelated OLED display 200 after deposition of one of the active color layers. The figure shows an array of banks 112 and wells 114 defining pixels of the display.
FIG. 3a shows a view from above of a substrate 300 for inkjet printing a passive matrix OLED display. FIG. 3b shows a cross-section through the substrate of FIG. 3a along line Y-Y′.
Referring to FIGS. 3a and 3b, a substrate is provided with a plurality of cathode undercut separators 302 to separate adjacent cathode lines (which will be deposited in regions 304). A plurality of wells 308 are defined by banks 310 constructed around the perimeter of each well 308, and leaving an anode layer 306 exposed at the base of the well. The edges or faces of the banks are tapered onto the surface of the substrate at an angle of between 10 and 40 degrees.
In the example shown, the cathode separators are around 5 μm in height and approximately 20 μm wide. Banks are generally between 20 μm and 100 μm in width and in the example shown have a 4 μm taper at each edge (so that the banks are around 1 μm in height). The pixels of FIG. 3a are approximately 300 μm2 but the size of a pixel can vary considerably, depending upon the intended application.
FIG. 4a shows a simplified cross section 400 through a well 308 filled with dissolved material 402, and FIG. 4b shows the same well after the material has dried to form a solid film 404. In this example the bank angle is approximately 15° and the bank height is approximately 1.5 μm. As can be seen a well is generally filled until it is brimming over. The solution 402 has a contact angle θc identified as angle 402a, with the plasma treated bank material of typically between 30° and 40° for example 35°; this is the angle the surface of the dissolved material 402 makes with the (bank) material it contacts. As the solvent evaporates the solution becomes more concentrated and the surface of the solution moves down the tapering face of a bank towards the substrate; pinning of the drying edge can occur at a point between the initially landed wet edge and the foot of the bank (base of the well) on the substrate. The result, shown in FIG. 4b, is that the film of dry material 404 can be very thin, for example of the order of 10 nm or less, in a region 404a wherein it meets the face of a bank. In practice drying is complicated by other effects such as the coffee ring-effect. With this effect because the thickness of solution is less at the edge of a drop than in the center, as the edge dries the concentration of dissolved material there increases. Because the edge tends to be pinned solution then flows from the center of the drop towards the edge to reduce the concentration gradiant. This effect can result in dissolved material tending to be deposited in a ring rather than uniformly. The physics of the interactions of a drying solution with a surface are extremely complicated and a complete theory still awaits development.
As previously mentioned, the bank and separator structures are typically formed from resist material, for example using a positive (or negative) resist for the banks and a negative (or positive) resist for the separators. Both these resists may be based upon polyimide and spin coated onto the substrate.
One problem with the aforementioned arrangement is that of providing adequate containment of the organic material deposited in the wells such that the wells are not flooded. At the same time, it is desirable for the organic material to spread out, or wet, a substantial part of the well so as to provide layers having an even thickness as otherwise the emission profile across the pixel will vary. Providing good organic film formation is critical to providing a high quality display. It has been found that film formation is affected by the composition of the solution of semi-conductive organic material to be deposited (the “ink”), the composition of the bank material, and the structure of the banks.
Having regard to the containment issue, the bank material can be modified to present a hydrophobic surface in order that the banks are not wetted by the solution of deposited organic material and thus assist in containing the deposited material within a well. This is achieved by treatment of a bank material such as polyimide with an O2/CF4 plasma as disclosed in EP 0 989 778. Alternatively, the plasma treatment step may be avoided by using a fluorinated material such as a fluorinated polyimide as disclosed in WO 03/083960.
However, it has been found that although good containment can be achieved with the aforementioned bank material, the organic material deposited in the wells can dry to form layers of uneven thickness.
One solution to the aforementioned problem is to modify the bank structure (well profile) so as to provide an undercut bank as described in GB-A-0402559.9. However, etching of organic resist material used for the banks can be difficult to control in order to arrive at an accurately formed well profile.
WO 03/065474 suggests that better films can be formed by overfilling the wells and describes the use of very high barriers to allow the wells to hold a large volume of liquid without the liquid overflowing to adjacent wells. However such structures cannot easily be formed by photolithography of the usual organic resist materials used for the banks.
Another problem associated with organic optoelectrical devices, such as those discussed above, is that in the resultant device the organic hole injecting layer can extend beyond the overlying organic semi-conductive layer providing a shorting path between the cathode and the anode at an edge of the well. This problem is exacerbated if the contact angle of the organic hole injecting composition with the bank material is too low.
One solution to the aforementioned problem is to modify the bank structure by, for example, providing a stepped bank structure which increases the length of the shorting path, thus increasing the resistance of the path resulting in less shorting. Such a solution has been proposed by Seiko Epson. However, as stated previously, etching of organic resist material can be difficult to control in order to arrive at an accurate and reproducible well profile.
Another problem with organic devices is that they tend to be sensitive to moisture and oxygen.
In light of this, the substrate preferably has good barrier properties for prevention of ingress of moisture and oxygen into the device. The substrate is commonly glass. However, alternative substrates may be used, in particular where flexibility of device is desirable. For example, the substrate may comprise a plastic as in U.S. Pat. No. 6,268,695 which discloses a substrate of alternating plastic and barrier layers or a laminate of thin glass and plastic as disclosed in EP 0 949 850.
The device is preferably encapsulated with an encapsulant on an opposite side to the substrate in order to prevent ingress of moisture and oxygen. Suitable encapsulants include a sheet of glass, or an airtight container, such as a metal can, as disclosed in, for example, WO 01/19142. These encapsulates are typically adhered to the substrate around the periphery of the device using a resin. Films having suitable barrier properties such as alternating stacks of polymer and dielectric, as disclosed in, for example, WO 01/81649, may also be used. A getter material for absorption of any atmospheric moisture and/or oxygen that may permeate through the substrate or encapsulant may be disposed between the substrate and the encapsulant.
While the aforementioned arrangements provide good protection against ingress of moisture and oxygen through the top and bottom of the device (i.e. through the encapsulant and the substrate in a direction perpendicular to the plane of the device layers) there is still a problem with lateral moisture and oxygen ingress through the sides of the device (i.e. through the sides of the device in a direction parallel to the plane of the device layers). This is particularly problematic at the periphery of the device where electrical connections of the electrodes exit the device for connecting to a power supply. These “exit tracks” usually pass out of the device at an exit region at the periphery of the device between the substrate and encapsulant where the encapsulant is adhered to the substrate.